In one type of projection system, a light valves is illuminated with an illumination beam that is at least approximately homogeneous across the active area of the light valve. For example, U.S. Pat. No. 5,625,738 describes a light tunnel for beam shaping and homogenization. Multi-reflections at the inner surfaces of the light tunnel, which are covered with a high reflection coating, provide an intensity distribution of the light that is approximately homogeneous at the exit of the light tunnel. In addition, the light tunnel transforms the illumination beam from a circular shape at the entrance of the light tunnel to a rectangular shape at the exit of the light tunnel.
Projectors based on color-sequential image formation only have one light valve, and the color-sequential beam is directed upon the single light valve. U.S. Pat. No. 5,448,314 discloses a method of field sequential colored illumination of a light valve used for projection. Within the method, light from a white light source is propagated through a transmitting periphery of a color wheel. The periphery includes a plurality of different color filters. Therefore, different colors are sequentially transmitted when the color wheel is rotated. The light valve is driven in such a way, that the actual image, formed by the light valve, corresponds to the actual transmitted color of the light used for illumination. If the color wheel is rotated fast enough and the light valve is accordingly switched fast enough, the human eye is not able to resolve the different images on the screen and the impression of a colored image is achieved.
FIG. 1 shows a known projector 101 that has a light tunnel 111 and that utilizes the field sequential colored illumination method. A white light source 115 (e.g., light emission component and reflector) emits light downstream within the projector 101 toward the light tunnel 111. In general, the progression of the light is referred to as a stream path. The light is focused down to a beam for entrance into the light tunnel 111. As the light beam is transmitted through the light tunnel 111, the light beam is transformed to have an intensity distribution that is at least approximately homogeneous. The shape of the light beam is adapted to the shape (e.g., rectangular) of an active area of a light valve 121, which is located further downstream.
The rectangular-shaped and homogeneous light beam, which exits the light tunnel 111, impinges on a color wheel 105. A plurality of color filters is provided on the color wheel 105, and each filter transmits a different wavelength range. Thus, the color wheel 105 transmits a predetermined wavelength interval, dependent on where the light beam impinges on the color wheel 105. Light that is not transmitted through the color wheel is reflected. The color wheel 105 is mounted on the axis 106 of a rotational motor 107. As the color wheel 105 rotates, different colors are sequentially transmitted.
As the light beam proceeds from the color wheel 105, the light beam is well prepared for being used as an input to the light valve 121, which is operated for field sequential colored illumination. A lens 117 is located downstream of the color wheel 105 and is used for imaging the exit 112 of the light tunnel 111 or the color pattern off a SCR technology based CW on the active area of the light valve 121. A prism construction 119 is interposed between the lens 117 and the light valve 121 to reflect the light beam to the light valve 121.
The light from the beam that is used for image formation is reflected by the light valve 121 in such a way that it is transmitted back through the prism 119. This transmitted light is propagating to a projection lens 123, which in turn projects the image on a screen (not shown). Light that is not used for image formation is reflected by the light valve 121 in such a way, that it is not propagated to the projection lens 123.
As shown in FIG. 1, the light tunnel 111 and the color wheel 105 are mounted separately to a housing 103 of the projector 101, independent from each other. Specifically, a fixation 109 for the motor 107, which rotationally supports the color wheel 105, is a different component than the fixation 113 for the light tunnel. In fact, the fixation 113 for the light tunnel 111 is well separated from the fixation 109 for the motor 107. However, for optical reasons, it is advantageous and in some applications important to place the entrance or the exit of the light tunnel 111 as close as possible to the color wheel 105.
Installation of the light tunnel 111 in the projector 101 requires a first set of accurate and therefore expensive adjustment procedures. Installation of the color wheel 105 in the projector 101 requires a second, different set of accurate and expensive adjustment procedures. If the housing 103 of the projector 101 changes, both adjustment procedures often have to be adopted to the new layout. Due to market requirements, such changes to the housing 103 happen quite often. Projectors for business applications or for consumer applications need to be small. One reason for the need to be small is that the small size of the projectors is one of the key selling factors. Such, small size complicates the assembly and adjustment of the light tunnel 111 independent from the color wheel 105.